There are several settings that affect the speed of display when you are inside DVDSP. The one that always gets me is the settings drop down in the menu editor - turn off the 'assign navigation continuously' option. This makes sure that every time you move a button when building a menu that links to all other buttons are created in a logical manner. However, even moving a button slightly results in a delay. It gets progressively worse as you ad more buttons. Turn it off and move them where you want, then remember to assign the navigation 'now' when you are done... if you don't then you won't be able to skip between the buttons when the disc is created.
Also, are you menus made with video assets? Do you have the 'motion' tab checked so that the menus start to 'move' as they appear? This will considerably slow you down, too.